Externalize gnome-split.desktop.

Do not generate the gnome-split.desktop at configure time anymore.
An classic file is easier to modify for possible contributors.
Removal of old and deprecated StatusIcon.

No more usable StatusIcon as per GNOME 3 design guidelines. This old
behaviour was actually not usable in GNOME 3 or in Unity desktops.
Remove the StatusIcon is a first step to start the refactoring of
GNOME Split to match GNOME 3 applications design.